1. Field of the Invention
The present invention relates to thermoplastic compositions based on a polyolefin and a vinyl aromatic polymer. More particularly, the present invention relates to thermoplastic compositions based on a polyolefin and a vinyl aromatic polymer endowed with an optimal combination of properties, including thermoformability, moldability, thermal resistance, modulus, resilience, resistance to naturally-occurring organic substances which tend to accelerate the degradation of some polymers, etc.
2. Description of the Background
Such particular combination of properties makes the compositions of the present invention particularly suitable for the production of shaped articles by any forming technique known for the thermoplastic resins, such as thermoforming, injection molding, forging, rotational molding and the like.
As is known, vinyl aromatic polymers are thermoplastic resins suitable for being transformed, under heating, into shaped articles by injection or extrusion molding. Said vinyl aromatic polymers have a fair tenacity, but they are not suitable for use in applications where a high tenacity associated with good chemical resistance is required.
A way to improve this deficiency in properties is to provide blends with other polymers which do not lack these properties to obtain a material exhibiting the desired combination of properties. However, this approach has only been successful in a few cases; generally, in fact, the blending results in the combination of the worst characteristics of each component, the overall result being a material of such poor properties as not to be of any practical or commercial value.
The reasons for this drawback are the fact that not all polymers are compatible with each other and, therefore, do not perfectly adhere. As a result, interfaces are formed among the components of the blend which represent weakness and breaking points.
More particularly, mixtures based on polystyrene, or generally on a vinyl aromatic polymer or copolymer, either as such or made impact resistant by grafting it onto a rubber, and onto a polyolefin, give rise to blends endowed with heterogeneous structure and mechanical weakness, due to the incompatibility between these two types of polymers.
Many attempts to solve the incompatibility problem have been made by using defined amounts of the two resins and by making use of a so-called compatibilizing agent.
Thus, for example, U.S. Pat. Nos. 4,386,187 and 4,386,188 describe the use of a styrene-butadiene-styrene block copolymer as compatibilizing agent between the polyolefins and a vinyl aromatic polymer.
Published European Patent Application 60, 524 and Japanese Patent Publication (Kokai) 49-28637/1974 describe the use of styrene-butadiene block copolymers to compatibilize blends of a polyolefin with a high impact polystyrene or styrenic resins.
Published European Patent Application 291,352 discloses polymeric compositions comprising a polyolefin and a vinyl aromatic polymer containing, as elastomeric component, small quantities of a vinyl aromatic monomer-conjugated diene linear block polymer, wherein, as a compatibilizing agent, a vinyl aromatic monomer-conjugated diene star-block polymer is used.
In English Patent 1, 363, 466 and in U.S. Pat. No. 4,020,025, hydrogenated styrene-butadiene di-block copolymers are proposed as compatibilizing agents for polyolefin/styrenic polymer blends.
In the published European Patent Application 60525 and U.S. Pat. No. 4,188,432, a hydrogenated styrene-butadiene-styrene block copolymer, i.e. a styrene-ethylene-butylene-styrene copolymer, is blended with styrenic and olefinic polymers.
In general, however, these approaches have not been industrially successful since the resulting blends, even if they are homogeneous, do not exhibit a satisfactory property combination or have other undesirable properties.
Laid-open European Patent Application 195,829 describes the use of an alpha-olefin copolymer grafted vinyl aromatic polymer, as a compatibilizing agent in the polyolefine-vinyl aromatic polymer blends.
The polymeric compositions disclosed in this European Patent Application comprise: at least 20% by weight (preferably between 20 and 90%) of a polyolefin; at least 5% by weight (preferably between 5 and 75%) of a vinyl aromatic polymer and from 2 to 15% by weight of the alpha-olefin copolymer grafted onto a vinyl aromatic polymer.
Also the compositions so obtained, however, do not exhibit a satisfactory combination of properties for all the uses they are intended for. In particular, an acceptable value of resilience (IZOD) may be obtained by using an excess of polyolefin; this increase, however, occurs to the detriment of the modulus property. By increasing, on the contrary, the amount of the vinyl aromatic polymer, the modulus of the composition increases but to the detriment of the resilience thereof which assumes values of poor use interest.